This invention is a catalytic process for the production of aromatic hydrocarbons with high yields.
A stable catalyst capable of being regenerated by treatment with hydrogen is used at low pressure in two sets of reactors arranged in parallel, one set of reactors being used to dehydrocyclize the hydrocarbons and the other set of reactors being regenerated by the hydrogen produced by the dehydrocyclization reaction.
The customary methods for aromatization of paraffins are based on the use of catalysts comprising a noble metal on a carrier, in particular catalysts containing from 0.2 to 0.8% by weight of platinum on a carrier of chlorinated alumina at 0.2-2% by weight. To be sufficiently stable, these catalysts must operate at a relatively high pressure (on the order of 30 atm.) in the presence of a hydrogen excess (6 moles H.sub.2 per mole of hydrocarbons) to limit the formation of coke. Unfortunately, the high pressure and hydrogen content thermodynamically and kinetically limit the dehydrocyclization reactions and promote the undesirable hydrocracking reactions.
An important improvement in these catalysts consisted in adding a second metal to the catalyst, which confers on the latter increased stability and the possibility of working at a lower pressure under conditions where the aromatization reactions are favored. Catalysts are now used which contain the coupled platinum-rhenium, or platinum-iridium, or platinum-tin, or platinum-germanium on the carrier of chlorinated alumina. The working pressures can be reduced to approximately 20 bars without affecting the cycle duration and with a remarkable increase in the yield of aromatic hydrocarbons and a reduction in the hydrocracking reactions.
Like the older catalysts, these catalysts are used in a fixed bed and after a cycle of a few months, they are regenerated by combustion of the coke followed by a treatment with a mixture of air and chlorinated compounds for dispersing the metals. After reduction by hydrogen, these catalysts are ready for use and have properties almost identical with those of a new catalyst.
Another important improvement in the process consisted of using catalysts in reactors with mobile beds. In the mobile bed reactors, the catalyst is continuously injected into the set of reactors where it remains for a period of time on the order of a few weeks. The catalyst is then withdrawn from the reactor, regenerated and then recycled to the reactor. All these operations are effected without stopping the production of aromatics. The reduction of the cycle to a few weeks makes it possible to lower the working pressure to about 10 atm., which has a favorable influence on the yields of aromatics and reduces the amount of hydrocracking which occurs in the process.
In all these processes, the catalysts comprised noble metals supported on chlorinated alumina. The catalysts become deactivated by formation of a coke which is a poorly hydrogenated polyaromatic species which limits the access of the hydrocarbons of the charge to the catalytically active sites. This coke can only be removed from the catalyst by combustion with small amounts of oxygen. The combustion operation is long and difficult since the combustion must be carefully controlled to avoid thermal degradation of the catalyst. The combustion operation must be preceded and followed by thorough purges to avoid explosions. For these reasons, the operation is effected as rarely as possible in spite of the advantages of a reduction of the pressure of the reaction which could be achieved by shortening the duration of the cycle.
The difficult regeneration of the fixed catalyst beds explains the advantage of using mobile beds with regeneration outside the reactors to achieve low working pressure. However, the duration of the cycle must still be equal at least to two weeks since it is not practical to circulate the catalyst too quickly through the bed thus requiring that the working pressure be not lower than 8-10 bars.
Other types of catalysts which are very effective in aromatization comprise platinum (from 0.1 to 1.5% by weight), optionally, a second metal such as rhenium, iridium, tin or germanium, and optionally sulfur, on a carrier comprising a crystalline aluminosilicate zeolite having a pore size larger than 6.5 Angstroms containing more than 90% alkaline cations. The zeolites can be the faujasites X and Y, the zeolite omega, the zeolite ZSM4 and the zeolite L. Zeolite L leads to specially interesting results in the aromatization of paraffins. Like the other catalysts, the zeolite supported catalysts become deactivated by formation of coke. However, we have unexpectedly observed that they can easily be regenerated by hydrogen, which makes it possible to shorten the duration of the regeneration considerably and to use them at low pressure with a spectacular improvement in yield, by using a new type of process.